You have to be sure the iCloud backup was purchased for your iCloud account. There's no sharing in a Family plan. My wife is in my family plan. But she has to have her own iCloud backup. This can be very confusing. Even though Family plans are intended to facilitate sharing, iCloud storage cannot be shared at this time.

You have to be sure the iCloud backup was purchased for your iCloud account. There's no sharing in a Family plan. My wife is in my family plan. But she has to have her own iCloud backup. This can be very confusing. Even though Family plans are intended to facilitate sharing, iCloud storage cannot be shared at this time.